Measurements of electrical impedance of the human body (bioimpedance) have been studied, in bioengineering, since 1960s. These measurements include forcing an alternating current (AC) through the body (usually at a frequency higher than 10 kHz to reduce interference with the electrical activity of nervous and muscular tissues), and sensing the voltage drop between two points.
Water and body fluids (blood, intra and extra cellular fluid, for example) provide the conductive medium. Several measures and studies have been conducted by applying this technique in different parts or regions of the body and using different frequencies to target different biological information (See for example, Deok-Won Kim, Detection of physiological events by impedance, Yonsei Medical Journal, 30(1), 1989). In numerous applications only the absolute value of the bioimpedance is to be determined because it is easier to calculate and provides much information. In other applications, both modulus and phase of the complex bioimpedance are measured.
It may be a relatively difficult to determine precise and reliable mathematical models of bioimpedance, particularly in the thoracic region. The main factors influencing electrical impedance in the chest are: the blood present in the heart and in the aorta; and the pleural fluids and the pulmonary circulation. Heart pumping, causing a varying spatial distribution of blood in the heart-aorta region, and respiration cause non-negligible variations of thoracic bioimpedance (i.e. the impedance of biologic tissues). From these variations it may even be possible to determine heart rate, breath rate, and evaluate cardiac output (volume of blood pumped by the heart for unity of time).
The measurements may be carried out using two or four electrodes, as schematically shown in FIG. 1. When using two electrodes, the measured impedance is the sum of the bioimpedance Zbody and of the contact impedance Ze at the electrodes. Generally, the impedance Ze disturbs the measurement of the impedance Zbody. Using a more refined four electrode setup, it may be possible to measure the impedance Zbody as a ratio between the measured voltage drop and the current forced through the body tissues with increased precision, because the measurement is no longer affected by the contact impedance Ze.
There is an increasing interest about methods of carrying out these measurements, because it is generally a non-invasive technique and may be correlated to a vast range of physiological parameters. Thus it may be seen as having a strong potential in many medical fields. Furthermore, simplicity of measurements, integratability, reduced size, and low cost of the equipment, may make the technique of measuring thoracic bioimpedance particularly suitable to be implemented in wearable or implantable health monitoring systems.
Generally, the voltage VZ(t) sensed on the electrodes is an AC signal that is modulated by the bioimpedance Z(t);VZ(t)=Z(t)I0 sin(ωt)With an AM demodulator it may be possible to obtain a base-band signal representing the modulus |Z(t)| of the impedance. The phase of Z(t) may be evaluated, for example, by measuring the delay between the input current and output voltage or with a phase and quadrature demodulation.
A block diagram of a typical circuit for measuring the impedance of a biologic tissue is illustrated in FIG. 2. An AC voltage generated by an oscillator is used to control a voltage-to-current converter that delivers a current Iz that is injected through the biologic tissue using two or four electrodes. The voltage on the biologic tissue is sensed, amplified, and AM demodulated for obtaining a base-band signal. The DC component Z0 and the AC component deltaZ of the obtained base-band signal are extracted using a low-pass filter LPF and a high-pass filter HPF and converted into digital form by an analog-to-digital converter ADC. This type of system is characterized by the presence of an instrumentation amplifier (INA) upstream the AM demodulator.
A drawback of such a signal processing path is the fact that the INA operates on the modulated input signal. For this reason, the known architecture of FIG. 2 requires either an INA of sufficiently large bandwidth and thus having a large current consumption, or the use of a relatively low frequency of the current that is injected in the body tissues for carrying out the measurement. This is a limitation, because INAs, especially low power consumption and low cost devices, usually have a relatively narrow bandwidth.
U.S. patent application publication No. 2009/234,262 discloses a device for measuring the impedance of a biologic tissue having a differential amplifier connected to the electrodes and an AM demodulator of the differentially amplified signal. This prior device has the same drawbacks of the prior device of FIG. 2.
Another known measurement system is depicted in FIG. 3, as disclosed in Rafael González-Landaeta, Oscar Casas, and Ramon Pallás-Areny, Heart rate detection from plantar bioimpedance measurements, IEEE Transactions on Biomedical Engineering, 55(3):1163-1167, 2008. AM demodulation is performed upstream the INA to increase the CMRR. The circuitry is fully differential and a differential stage with coupled amplifiers is used as first stage of the voltage drop on the electrodes. A high pass filter HPF and amplifier stage is used for extracting the AC components of the signal, deltaZ, that in many applications (for example thoracic bioimpedance measurement), including important physiological information.
By resuming, in the cited prior devices, there is an input amplification stage for amplifying (and, eventually, filtering) the signals collected on the electrodes and, downstream, a demodulation circuit for extracting the base-band components thereof.
U.S. Pat. No. 4,909,261 discloses a device for measuring impedances of biologic tissues that has two pairs of electrodes coupled through transformers to a circuit. The circuit applies a same AC voltage to the electrodes. The device also includes as many differential AM demodulators, each coupled to a respective pair of electrodes through a respective transformer. A circuit combines the AC and DC components of the two AM demodulated signals for measuring the impedance of the biologic tissue.
Each AM signal to be differentially demodulated is collected on the same pair of electrodes used to force current throughout a respective portion of the biologic tissue. Therefore, this prior device combines two demodulated AM signals not pertaining to the same portion of biologic tissue. Moreover, the presence of transformers may not make it suited for wearable applications.
A device that does not require any differential amplifier of the sensed voltage on the electrodes is disclosed in prior Italian patent application No. VA2010A000017, the applicant of which is the same as the present applicant. The device includes two single-ended AM demodulators respectively of the voltages towards ground of two electrodes, a differential amplifier of the base-band demodulated single-ended voltages, and a filter for extracting the DC and the AC components of the differential base-band voltage. An output circuit is adapted to generate an output signal representative of the impedance corresponding to the DC component of the base-band voltage.